Let $\vec{a},\vec{b},\vec{c}$ be three non-zero vectors, no two collinear. If $\vec{a}+\vec{b}$ is collinear with $\vec{c}$ and $\vec{b}+\vec{c}$ is collinear with $\vec{a}$, then $\vec{a}+\vec{b}+\vec{c}$ is equal to